1. Field of the Invention
The invention relates to inspection of microstructures, particularly for detection of defects in partially-fabricated microcircuits with the aid of a charged-particle-beam system.
2. The Prior Art
Various techniques are used to inspect for defects in microstructures such as partially-fabricated microcircuits. For example, optical inspection systems create an image of a microcircuit which is inspected for anomalies. However, such images have insufficient resolution to enable identification of the smallest features, offer insufficient distinction of defects which are electrically significant from those which are not, and have insufficient depth of focus for detection of sub-surface defects. Charged-particle-beam inspection systems have advantages over optical inspection systems when inspecting microcircuits fabricated with critical-dimension technology of 0.35 micron and smaller. Charged-particle-beam inspection has sufficient resolution to image small features such as contact holes, gates, and polysilicon lines, and can be used to detect killer defects based on voltage contrast. Floating conductors and conductors connected to n-diffusion regions should have higher or lower voltage than grounded conductors and conductors connected to p-diffusion regions. In a voltage contrast image, the latter typically appear darker than the former. An electrical defect can be identified in a voltage-contrast image if it causes a feature to appear brighter or darker than expected.
However, it is difficult to obtain a good voltage-contrast image of a microstructure having a high aspect ratio, such as the bottom of a contact hole which is deep relative to its width. While a voltage-contrast image normally shows obvious contrast differences between structures connected to ground, n-diffusion regions, p-diffusion regions, and gate regions, high-aspect-ratio structures do not. Instead, the bottom of a high-aspect-ratio structure appears in low contrast due to obstruction of secondary electrons by the side walls of the structure and consequent charging-up of the side walls.
An example of such a high-aspect-ratio structure is a contact hole of a wafer in an intermediate stage of fabrication. After preparing structures such as grounded regions, n-diffusion regions, p-diffusion regions and gate regions, they are covered with dielectric and contact holes are formed in the dielectric at appropriate locations so that conductors of a subsequent metal layer can make electrical contact with these regions. Because of the high aspect ratio of the contact holes, a voltage contrast image obtained using a high beam current has insufficient contrast to distinguish the regions.
Charged-particle-beam systems, such as scanning-electron microscopes in critical-dimension-measurement systems, can be operated at very low beam current for contact-hole imaging to prevent charging of the side wall. However, this imposes a limit on throughput of the system and results in diminished voltage contrast because beam current is insufficient to charge up the structures of interest. Imaging is also slow due to shot noise (current fluctuation caused by the discrete nature of electron charge).
U.S. Pat. No. 5,493,116 describes electron-beam imaging of high-aspect-ratio structures such as contact holes using two signal detection sub-systems, one optimized for imaging at the top and another optimized for imaging at the base of sub-micrometer structures. Signals produced by the detection sub-systems are combined to produce an image resembling extended focus images obtained with confocal optical microscopes.
Improved methods and apparatus are needed for detection of defects in microstructures and especially in semiconductor wafers carrying portions of microcircuits in fabrication.